This invention relates to a method of limiting maximum charging and discharging current values based on the state of charge or remaining capacity of the battery, and for example, relates to a method of controlling battery current limiting to limit maximum current through batteries included in a power source apparatus for powering the driving motor a car.
Batteries have the property that their electrical characteristics degrade if discharged at high current when their state of charge is low, or if charged with high current when their state of charge is high. For this reason it is important to limit maximum charging and discharging currents for batteries in a power source apparatus that, for example, powers a car driving motor. If maximum current is not limited, rapid car acceleration when the state of charge of the battery is low can cause battery voltage to suddenly drop due to high discharge current. In addition, sudden braking when the state of charge of the battery is high can cause battery voltage to rise rapidly. In either case, degradation of battery characteristics is the detrimental result. In particular, since batteries of a car power source apparatus are charged and discharged with very high currents, battery life becomes extremely short if maximum currents are not limited. This is especially important for a power source apparatus used in applications that require high battery output such as cars or automobiles, bicycles, and tools
Consequently, for high current charging and discharging without degrading battery electrical characteristics, it is important to limit discharging current when the state of charge of the battery becomes low to prevent over-discharging. Further, it is important to limit charging current when the state of charge of the battery becomes high to prevent over-charging. This is because over-discharging and over-charging reduce battery lifetime. Therefore, it is important to accurately detect the state of charge (SOC) of the battery and determine charging and discharging current limit values from the detected state of charge. Otherwise, if there is error in the detected state of charge, over-charging or over-discharging can markedly degrade battery characteristics.
Incidentally, the state of charge or remaining capacity of a battery is determined by subtracting battery discharge capacity from charge capacity. Battery charge capacity is computed from the integral of the charging current considering charging efficiency. Discharge capacity is computed from the integral of discharging current. Therefore, state of charge of the battery is computed during discharge by subtracting discharge capacity from the state of charge prior to discharge. Similarly, during charging, state of charge of the battery is computed by adding charge capacity to the state of charge prior to charging. By this method, the state of charge of the battery can be computed during charging and discharging. However, the state of charge computed by this method gradually accumulates error. Consequently, the state of charge computed from the integration of charging current and discharging current is not always in agreement with the correct state of charge of the battery. This is because factors such as the amount of discharge current and temperature can be sources of error.
In addition, there is also a method of determining the state of charge by measuring battery voltage, but this method as well cannot always determine the state of charge accurately. It is well known that a different battery voltage can be measured even at the same state of charge depending on factors such as the battery's charge-discharge history. Further, since there is a region where voltage changes little as a function of the state of charge for batteries such as nickel hydrogen and nickel cadmium batteries, it is difficult to estimate state of charge based on battery voltage alone.
In this manner, accurately determining the state of charge of the battery is difficult, and even at the same voltage and current values, the amount of usable power differs depending on factors such as state of charge and battery temperature. In particular, when the commonly described “memory effect” occurs, an actual decrease in battery capacity results, and battery state of charge determination becomes even more difficult. The memory effect is a phenomenon that occurs when a battery such as a nickel cadmium battery or nickel hydrogen battery is put through charge-discharge cycles with shallow discharge (low discharge levels not approaching full discharge). When a battery in this condition is deeply discharged, discharge voltage drops temporarily. Because the state of charge of the battery changes due to the memory effect, an accurate value of the state of charge cannot be estimated. If the state of charge of the battery is not determined accurately, battery over-load can occur during charging and discharging, and this can be a cause of marked reduction in battery lifetime. Meanwhile, change in the state of charge of the battery can also result from battery self-discharge. Because of these factors, estimation of the state of charge of the battery is difficult, and obtaining an accurate value of the state of charge is extremely problematical (see Japanese Patent Application Disclosure SHO 56-126776 (1981)).